PROJECT SUMMARY The major objective of this research program is to understand how networks of neurons communicate to produce visual perception. This knowledge will be useful to bioengineers and clinicians in the development of new devices and treatments for patients with perceptual disorders. Understanding how vision works in rhesus monkeys, an animal whose vision is similar to humans', will bring us closer to understanding the physiology of human vision in states of health and disease. The Specific Aims of this study target three different components of how the primary visual cortex (V1) contributes to visual contrast sensitivity. All of the Aims use a similar methodology: a novel, reversible system for neuronal inactivation combined with simultaneous quantitative behavioral measurement. These measurements will reveal how signals are routed through the visual system to control behavior. The first Aim is to determine how much of the baseline noise in V1 contributes to the variability of perceptual reports. This Aim will be achieved by suppressing V1 activity in a monkey trained to report the presentation of a low-contrast visual stimulus. On some trials, no stimulus is shown and the monkey must guess whether one was. A key question is whether suppression of V1 activity prevents the monkey from guessing that a stimulus was presented when none, in fact, was. The second Aim is to determine whether the visual pathways that bypass V1 are sufficient to mediate the monkeys' behavioral reports. We will achieve this Aim by testing the monkeys' ability to detect stimuli designed to activate the neural pathways that bypass V1. If the monkey can see these stimuli when activity in V1 is suppressed, and if the predictions of control experiments are confirmed, we will know that the pathways that bypass V1 can mediate contrast detection. The third Aim is to determine whether aspects of V1 activity other than the feedforward volley are necessary for stimulus detection. We will achieve this Aim by measuring the monkeys' threshold for detecting electrical microstimulation of area V2 when activity in V1 is suppressed. Together, completion of these Aims will answer important and outstanding questions about the role of area V1 in stimulus detection. These questions cannot be adequately addressed with classical techniques, and the inactivation technique that we will use to complete these Aims is an important technical contribution of the proposed work.